Hi there @Possolution,
Welcome to the Community!
I did some digging into this, and it seems like there are many plugins that block Zapier's attempts to connect to WordPress. The standard recommendation is to disable security-related plugins like JetPack and test your connection following the systematic disabling of the plugins.
Although, there are many other plugins that could also cause such an error, some users pointed out that disabling the Yoast SEO function 'REST API: Head endpoint' did the trick for them. Yoast is a very popular plugin, so this could be a good one to share if your customer is having trouble. Here's how to disable it:
https://yoast.com/help/how-to-disable-or-enable-the-rest-api-endpoint-in-yoast-seo/
In addition, disabling "Restrict Content" and the WPassword plugin also fixed the issue for some users.
Hopefully, this helps!